1. Field
The following description relates to a light-emitting diode (LED) driver apparatus, and more particularly, to an LED driver apparatus that supplies a stable output voltage and a stable output current at an initial driving.
2. Description of the Related Art
A liquid crystal display (LCD) is thinner and light in weight than other types of display apparatuses and has lower driving voltage and power consumption than the other types of display apparatuses. Therefore, the LCD is widely used. However, the LCD is a non-emitting device that cannot self-emit light and requires an additional backlight to supply light to an LCD panel.
A cold cathode fluorescent lamp (CCFL), a light-emitting diode (LED), or the like is mainly used as a backlight source of the LCD. The CCFL uses mercury and, thus, may cause an environmental pollution. Also, the CCFL has a slow response speed and a low color representation, and is inappropriate for making the LCD panel light, thin, short, and small.
The LED does not use an environmentally harmful material and, thus, is eco-friendly and enables impulse driving. Also, the LED has a high color representation, arbitrarily adjusts light amounts of red (R), green (G), and blue (B) diodes to change a luminance, a color temperature, etc., and is appropriate for making the LCD panel light, thin, short, and small. Therefore, the LED has been widely used as a backlight source of the LCD panel or the like.
LED arrays include a plurality of LEDs and are operatively connected to one another. When LED arrays are used in the LCD backlight unit, a driving circuit is needed to provide a constant current to each of the LED arrays, and a dimming circuit is also needed to adjust brightness and color temperature arbitrarily or compensating for overheating.
An LED driver apparatus compares a target voltage to drive all of the LED arrays with a driving voltage (for example, a feedback voltage) that is applied to the LEDs to control a feedback on the driving voltage. A feedback operation of a conventional LED array will now be described with reference to FIG. 6.
FIG. 6 is a graph illustrating waveforms of an initial driving voltage of a conventional LED driver apparatus, a comparison voltage, and a driving current.
Referring to (a) and (b) of FIG. 6, in order to turn on all of LED arrays at an initial stage to drive LEDs, a driving voltage higher than a target voltage is applied to the LED arrays. If the high driving voltage is applied to the LED arrays, all of the LED arrays are turned on, and, thus, a constant current flows through the LED arrays.
After the high driving voltage is applied to the LED arrays, a feedback control is immediately performed to compare the target voltage with a feedback voltage to output a comparison voltage. However, if the feedback control is immediately performed when the high driving voltage is applied to the LED arrays, the comparison voltage is lowered to a minimum voltage to weaken voltage boosting due to the higher driving voltage than the target voltage. Therefore, the voltage driving the LEDs is rapidly lowered, as a result, the driving voltage becomes lower than the target voltage. Subsequently, the comparison voltage increases in order to strengthen voltage boosting. Also, weakening and strengthening of boosting are repeatedly performed to converge the driving voltage into the target voltage due to a feedback time delay.
However, if weakening and strengthening of boosting are repeatedly as described above, a distortion of the constant current occurs as shown in (c) of FIG. 6.